CHEM 215 EXAM 1

analytical chemistry

branch of chemistry that deals with separation, identification, and quantification of target components in a sample

sample

portion of material selected from a larger quantity of material

analyte

the target compound to be determined quantitively or qualitatively in a sample

matrix

the components of a sample other than the analyte

qualitative analysis

concerning the identity of the analyte (what)

quanitative analysis

concerning the amount of analyte (how much)

density

mass/volume; g/L

volume=

length³, 1 mL=1cm³

Molarity M

mol/L

wt %

(100%)(mass of substance)(mass of mixture or solution) ; (g/g)(100)

Conversion between wt% and molarity of solutions requires what?

density of the solution

ppm

(grams of substance)/(10^6 grams of mixture or solution)

when does 1 g of solution = 1 mL?

for dilute aqueous solutions at room temperature

ppm at room temperature dilute aqueous solution

[(g solute)/(g solution)](10^6) = mg/L

ppb at room temperature dilute aqueous solution

[(grams solute)/(grams solution)](10^9) = ug/L

dilution equation

(M1)(V1)=(M2)(V2)

leading zeros are…

not significant

trailing zeros are significant if

a decimal is present

The last digit of a measurement is called the…

least significant digit (LSD)

least significant digit LSD

the digit with uncertainty

Round off rules: <5

round down

Round off rules: >5

round up

Round off rules: =5

round to nearest even integer (0.01405 → 0.0140) (0.01415 → 0.0142)

Addition and Subtraction sig figs

the number with the LSD furthest to the left determines the LSD in answer

multiplication and division sigfigs

the number with the smallest # of sigfigs determines the # of sigfigs in the answer

LSD is preserved in measurement

When taking the log of a number, how many sigfigs should the answer have?

the # of digits after the decimal in the answer equals the # of sigfigs in the number

When taking the antilog (exponential) of a number, how many sigfigs should there be?

The # of sigfigs in the answer equals the number of digits after the decimal place in the number

Square roots or cube roots should have how many sigfigs?

Answer should have the same # of sigfigs as the number

At the endpoint of a titration…

mol of acid= mol of base

Accuracy

how close an experimental measurement is to the true value

Precision

how reproducible a measurement is over multiple measurements

systematic error

experimental error due to causes which can be corrected ; affects accuracy but precision may or may not be affected

random error

experimental error die to causes which can not be corrected; does not effect accuracy but defines smallest possible uncertainty in a measurement

calculating standard deviation

sq rt ((x1-xm)² + (x2-xm)² + (x3-xm)²)/n-1)

For tests report x and s with…

the same LSD as measurements

relative uncertainty

s/x (may be expressed as a percent)

confidence interval

the range of values about x in which there is a given confidence level (probability) in which the true mean u lies

confidence interval =

+- (t*s)/ sq rt N

s- SD

t- reference table

n= # replicants

confidence intervals are usually rounded to the…

same LSD as measurements

Grubbs test

for rejection of single bad data

G data= |(questionable value-x)|/s

if G calc> G table, value should be rejected

calibration curve

a graph of the response of an instrument vs the amount (concentration) of the analyte

Group 1 Cations charge: Li, Ka, K

+1

Group 2 cations charge Mg, Ca, Ba

2+

Al

3+

In

3+

Tl

+ or +3

Sn

2+

Pb

2+

Sn

4+

Pb

2+ or 4+

Bi

3+

Group 7 anions charge F, Cl, Br, I

-1

Group 6 anion charge O, S

2-

OH

-1 hydroxide

NO3

-1 nitrate

SO4

2- sulfate

Cl04

-1 perchlorate

Ch3COO

-1 acetate

CO3

2- carbonate

HCO3

-1 bicarbonate

C2O4

2- oxalate

reaction quotient Q

prod/ react ;

C^cD^d/A^aB^b

At equilibrium

Q=K= equilibrium constant

K depends on the temperature, but not on

concentrations

For a pure solid or liquid, what is the ratio and do they appear in expression of Q or K?

ratio=1, NO

Solubility (insoluble salt in water)

AgCl (s) ←> Ag+ + Cl - Ksp= [AG+][Cl-]

The reactant is always an insoluble salt, a pure solid, no term appears in denominator

Acid Dissociation (Acid in water)

HF ←> H+ + F- Ka= ([H+][F-])/[HF]

Water Dissociation

H2O ←> H+ + OH- Kw= [H+][OH-]

Base dissociation (base in water)

NH3+ H2O ←> NH4+ + OH- Kb= ([NH4+][OH-])/[NH3]

Complexation (metal and ligand in water)

Ag+ + 2NH3 ←> Ag(NH3)2+ Kf= [Ag(NH3)2+]/[Ag+][NH3]²

Invert the equilibrium equation A→ B B→ A

K’= 1/K

Multiple the equilibrium eq by a constant

K’=K^c

Add 2 equilibrium eq to produce a third equation

Add A→ B and C→ D to get A+C → B+ D

K’=K1K2

Pka=

-log K

K=

10^-pka

common ion effect

the solubility of a salt in reduced by the presence of an excess amount of one ion; used to ensure the analyte is quantitively precipitated

gravimetric analysis

any method in which the mass of a substance is used to determine the amount of the analyte

Goals of Precipitation Analysis for gravimetric analysis

Analyze the amount of ion in a homogenous solution

1. Quantitative precipitation

2. easily filtered product (large crystals)

3. pure product

4. known and constant composition of the product

Quantitative precipitation can be achieved by

• choose product with very small molar solubility

• adding excess precipitation ion

Easily filtered product goal requires large crystals; how can this be done?

• avoid extreme supersaturation

• encourage coagulation of small particles

how can you avoid extreme supersaturation?

want growth of nuclei, but not formation of many nuclei

1. mix in hot solution

2. mix slowly with vigorous stirring

3. work with dilute solutions

4. using homogenous precipitation (generate precipitating species by a slow reaction)

How can you encourage particle coagulation?

1. presence of an electrolyte to screen the surface charge on particles (surface charge inhibits coagulation)

2. digestion (heating/ aging) to dissolve smaller particles (heat) & grow larger ones (age)

pure product goal requires what?

• removing interfering ions prior to precipitation or kept in solution with a masking agent

Knowing the product composition is pure and stable requires?

• the product precipitate must be dried to a constant composition

• want low temp drying

  • removes water & volatile electrolyte (added to prevent breaking up of precipitate)

  • High temp changes product composition

For gravimetric analysis calculations, if a pure product is isolated and weighed, then…

g product → mol product → mol analyte → desired final units

Why is all equilibria involving ions shifted to the direction of the higher ion [] as the total ion [] increases?

ion in solution are surrounded by water molecules and an ionic atmosphere of oppositely charged ions. The presence of ion atmosphere reduces the attraction between any given anion and cation.

• the solubility of insoluble salts increases

• weak acids and bases dissociate more extensively

Activity

Aca= [ion]Yi

activity coefficient Yi

unitless number that varies from 0 to 1.

Value depends on both total [ion] and identity of the ion.

ionic strength u

u= sum(CiZi²) / 2

Ci= molar conc.

Zi= charge of ion

As u increases, what happens to Y?

Y decreases towards 0

For a given u, an ion with a ________ absolute charge has a smaller Y

larger

For neutral compounds, Y=

Y=1

inert ions in solubility calculations

inert ions control the ionic strength

Common ions in solubility calculations

common ions control the ionic strength & affect the solubility of salt

Kw

water dissociation constant H2O (l) ←> H+ + OH-

Kw=AhAoh=[H+]yh[OH-]yoh

H2O+ H2O ←>

H3O+ OH -

at 25 degrees celcius, Kw=

1.0:10^-14

in pure water, [H+]=

[OH-]

Kw=[H+][OH-]=[H]²

[H+]=[OH-]= 1.0×10^-7

pH =

-log(Ah)

-log[H+]

pOH=

-log(Aoh)

-log[OH-]